ABSTRACT This letter presents a high‐ Q tunable waveguide (WG) filter with constant absolute bandwidth (CABW). By adjusting the penetration depth of a single dielectric tuning element into the filter, frequency tuning is readily obtained without a complicated mechanism. Modified cavity perturbation theory (CPT) is adopted for the first time to deduce the quantitative relationship between the penetration depth and resonant frequency. At the same time, the tuning element also plays a vital role in realising innovative inter‐resonator (IR) coupling and input‐output (IO) coupling structures, combined with the configuration of double septa to ensure the CABW and IO port impedance matching. Finally, to validate the proposed concept, a prototype of a fifth‐order inline tunable WG filter is implemented, which has a tuning range of 8% from 18.2 to 19.7 GHz and the 1‐dB bandwidth variation is maintained within 315 ± 15 MHz. Measured results match the simulated ones very well, which show that the insertion losses are less than 0.6 dB and the return losses are better than 20 dB for all tuned passbands. The high performance of the proposed tunable WG filter makes it very suitable for the applications of 5G/6G millimetre‐wave backhaul or satellite telecommunication.